In 2000, the study conducted by Perou et al. established four intrinsic molecular subtypes of breast cancer based on gene expression data obtained from complementary DNA (cDNA) microarrays that analyzed 8102 genes from 65 human breast cancer specimens derived from 42 different patients: Luminal, HER2-enriched, Basal-like, and Normal Breast-like [80]. Here, ERBB2 is linked to breast cancer.